I. Field of the Invention
The present invention pertains to adhesives for bonding polyolefin film, polyolefin film employing this adhesive and methods for applying the polyolefin film with adhesive to a container.
II. Discussion of Background References
Polyvinyl chloride is currently employed for shrink wrap films. However, use of polyvinyl chloride is diminishing due to actual or perceived environmental concerns. Thus, it would be desirable to find a substitute for polyvinyl chloride. Polyolefins such as polypropylene or polyethylene would be desirable alternatives but they are difficult to bond. Polypropylene is particularly difficult to bond because it has a low surface energy (i.e. 27 dynes/cm.). Tensile shear during heat shrinking of polypropylene causes overlapping film ends to separate from one another. This is particularly a problem in commercial use because ovens used to heat shrink the polypropylene film about a product typically have varying temperatures at different parts of the oven. Thus, the adhesive has to withstand local hot spots of typical 230.degree.-250.degree. F. In shrink wrap applications, where a label is attached to a container and then the container is spun such that the label wraps around the container, an adhesive needs to be able to stick on contact because typical commercial units wrap from 100 up to more than 1000 containers per minute.
There are basically three ways to apply a label to a container: wrap, mandrel and sleeve.
Wrap methods are disclosed by U.S. Pat. Nos. 4,844,760; 4,781,785; 4,694,633; 4,544,431; and 4,574,020 incorporated herein by reference. In the wrap method the label is attached to the container and then the container is spun to wrap the label about the container. The label is then heat shrunk if necessary.
Mandrel methods are disclosed by U.S. Pat. Nos. 4,496,409 and 4,366,016 incorporated herein by reference. In the mandrel method the label is wrapped about a mandrel and then drops off the mandrel to surround a container. The label is then heat shrunk to snuggly fit the container.
Sleeve methods are disclosed by U.S. Pat. Nos. 4,454,181; 4,562,684; and 4,806,187 incorporated herein by reference. By this method, a sleeve is cut from a pre-formed roll of flat tubular film. The sleeve is placed about the container and then heat shrunk to snuggly fit the container.
Elastomeric polyurethanes are known adhesives. U.S. Pat. Nos. 3,001,971 and 2,871,218 disclose some polyurethanes. Polyurethanes may include additives. U.S. Pat. No. 4,301,053 discloses an aqueous carboxyl functional urethane dispersion crosslinked with XAMA-7 (a trifunctional aziridine). Improved adhesion to plastic substrates was achieved by incorporating a small amount of n-methyl pyrrolidone into the formulation.